Conventionally, in an EGR control of an EGR system of an engine, there is the one that sets a target EGR rate for each operational state constituted of, for example, an engine speed, an engine load such as a fuel injection rate and an acceleration opening degree, and conducts a control of an opening degree of an EGR control valve to attain the target EGR rate. However, such a control cannot cope with a change in air density during an operation on, for example, a highland, and therefore smoke and exhaust fine particles (PM) are increased due to a too large amount of EGR, or a sufficient NOx reduction effect cannot be obtained due to a too small amount of EGR in some cases. If EGR is carried out when the fuel injection rate is rapidly increased at the time of rapid acceleration or the like, an excess air ratio is sharply reduced, and there arises the problem of smoke and PM being increased. Due to this, EGR is conventionally reduced at the time of rapid acceleration or the like. However, with this method, EGR is reduced more than necessary, or the EGR amount becomes too large due to overshooting on the other hand, which makes it impossible to cope with the difference in the environmental condition or cope with the circumstance of the transient operation, and therefore there arises the problem that a sufficient effect cannot be obtained. As the solution to these problems, for example, Japanese Patent Laid-open No. 9-53519 discloses an exhaust gas recirculation control apparatus of an internal combustion engine.
According to the above-described Japanese Patent Laid-open No. 9-53519, the operational state of the engine is detected to set the target EGR rate, then, an intake air flow rate is detected, and the target EGR amount taken in a cylinder is set based on the target EGR rate. The valve opening degree of the EGR control valve is controlled based on the target EGR amount. As a result, even if the environment changes and an amount of intake fresh air changes, the control to attain the optimal EGR rate can be performed. Predetermined advance processing is performed for the set target EGR amount to set a command EGR amount, and the EGR control valve opening degree is controlled based on this, whereby the influence of delay in response is also reduced at a transient operation, and a control to attain a favorable EGR rate can be performed.
However, according to the above-described method, the intake air amount is detected, and the EGR valve opening degree is controlled based on this. Consequently, a delay occurs at the time of rapid acceleration and to a rapid load variation, and there is the fear that a favorable EGR control is not performed.